This invention relates to hydrocarbon conversion processes which use intermediate pore size molecular sieves. In particular, the invention relates to the selective conversion of normal and slightly branched paraffins into a lower molecular weight product mixture containing substantial amounts of olefins.
Straight chain and slightly branched chain paraffins are components of both natural and synthetic hydrocarbonaceous feeds. The presence of these compounds is often undesirable. To high-boiling oil fractions they give high pour points and to lower boiling fractions they give low octane numbers.
Many paraffin removal techniques have been reported in the literature. The product of these processes, where crystalline zeolites are used, is often a light paraffinic gas. This gas is not particularly reactive or valuable so it is burned in the refinery or used as LPG fuel. (U.S. Pat. No. Re. 28,398, Chen et al., Apr. 22, 1975--dewaxing with ZSM-5-type zeolites.) In other hydrodewaxing processes, the zeolite performance is improved by controlling the activity as measured by the alpha index (U.S. Pat. No. 4,247,388, Banta et al., Jan. 27, 1981).
Processes which produce more reactive and valuable olefins have been proposed. These processes require low pressures to prevent the catalyst from hydrogenating the product olefins; ZSM-5-type catalysts are used. (U.S. Pat. No. 4,171,257, O'Rear et al., Oct. 16, 1979; U.S. Pat. No. 4,251,348, O'Rear et al., Feb. 17, 1981; U.S. Pat. No. 4,282,085, O'Rear et al., Aug. 4, 1981.)
Further research into the characteristics of intermediate pore size materials has revealed that low aluminum content materials, such as those disclosed in U.S. Pat. No. 4,061,724, Grose et al., Dec. 6, 1977, and U.S. Pat. No. Re. 29,948, Dwyer et al., Mar. 27, 1979, have catalytic activity and can produce olefins from paraffins. See U.S. Pat. No. 4,309,275, Mulaskey, Jan. 5, 1982; U.S. Pat. No. 4,309,276, Miller, Jan. 5, 1982.
We have discovered a surprising relationship between pressure and silica:alumina mole ratio of the zeolite for olefin preparation reactions which use intermediate pore size materials. Our discoveries lead to a highly efficient process for producing olefins from paraffins.